Wednesday, December 31, 2014

Why is the Programming Resolution Supplemental Characteristic Listed as an Average?

Hello everyone!

Happy New Year!  This is our last post of 2014 so we wanted to wish all of our readers a Happy New Year.  Today I am going to talk about a question that I have been asked a few times lately.  In many of our power supplies, we list our Programming Resolution as an average number.  Many people want to know why we do it this way.

Look at the below snippet from our 664xA DC Power Supplies Supplemental Characteristics:


You can see that that it is clearly stated as an average.

The simple answer to this question is that this is because of calibration.

The more complex answer is that we use a DAC to control the output setting of the power supply.  A certain number of DAC counts is going to represent zero to full scale on the output of the supply.  For simplicity's sake, lets assume that we are using a 12 bit DAC for a power supply that goes to fifty volts.

In an ideal world where calibration is not necessary:
A 12 bit DAC gives us 2^12 or 4096 total counts.
The step size (programming resolution) of the 50 volt power supply would be 50/4096 or 0.0122 volts.

We do not live in an ideal world though so we have to disregard some DAC counts because of how the unit calibrates. We also generally let you program a little bit above the maximum settings (usually something like 2%).  Zero volts is not going to be zero DAC Counts and 50 V is not going to be 4096 DAC counts.  For our example, lets say that the minimum that we disregard 20 counts at the top and bottom (40 total counts) and the maximum we disregard is 120 counts (240 total counts) at the top and bottom.  In this scenario:

Minimum step size = 50/(4096-40) = 0.0123 V
Maximum step size = 50/(4096-120) = 0.0130 V

For our Supplemental Characteristic, we would take the average of those 2 numbers.   This gives us 0.01265 V.

The big question is how would I know what the programming resolution is for my particular unit.  I spent about half of yesterday trying to figure that out and I'm still working that out myself.  The best solution that I have right now is to hook a DMM to the output and slowly increment my output to see when it flips to a new setting.  I need to experiment on this though.  If any readers have a better idea, please let us know in the comments.  The fact of the matter is that the error is pretty small and to be safe, any error due to being in between DAC counts is included in our Programming Accuracy specification.

Well that is all for 2014. I hope that everyone has a safe and happy 2015.  See you next year!

Matt

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